This invention relates to processing of thin semiconductor wafers such as slices of semiconductor silicon and, more particularly, to improved method and apparatus for polishing wafers having uniform flatness of the polished surface, the improved polished wafer flatness is achieved through adjusting the contact surface profile of the wafers as carried by a pressure plate in contact with a polishing surface supported by a turntable which exhibits a thermal and mechanical bow from its axis of rotation to its edge.
Modern chemical-mechanical semiconductor polishing processes are typically carried out on equipment where the wafers are secured to a carrier plate by a mounting medium, with the wafers having a force load applied thereto through the carrier by a pressure plate so as to press the wafers into frictional contact with a polishing pad mounted on a rotating turntable. The carrier and pressure plate also rotate as a result of either the driving friction from the turntable or rotation drive means directly attached to the pressure plate. Frictional heat generated at the wafer surface enhances the chemical action of the polishing fluid and thus increases the polishing rate. Such polishing fluids are disclosed and claimed in Walsh Et Al. U.S. Pat. No. 3,170,273. Increased electronic industry demand for polished semiconductor wafers has promoted need for faster polishing rates requiring sizable loads and substantial power input for the polishing apparatus. This increased power input appears as frictional heat at the wafer surface. In order to prevent excessive temperature buildup, heat is removed from the system by cooling the turntable. A typical turntable cooling system consists of a coaxial cooling water inlet and outlet through a turntable shaft along with cooling channels inside the turntable properly baffled to prevent bypassing between inlet and outlet. However, it has been found that a major cause of distortion of wafer surfaces is resulting from a bow distortion of the turntable supported polishing surface substantially resulting from the heat flow from the wafer surface to the cool water which causes the top surface of the turntable to be at a higher temperature than the bottom surface. This temperature difference results in a thermal expansion differential causing the turntable surface to deflect toward the cool surface from the axis of rotation to the outside edge.
The wafer carrier is thermally insulated from the pressure plate by a resilient pressure pad. Therefore, the carrier approaches thermal equilibrium at a substantially uniform temperature and remains flat. The difference in curvature between the plane defined by the wafers and the bowed surface of the turntable results in excessive stock removal toward the center of the carrier causing non-uniform wafer thickness and poor flatness. This lack of uniformity and flatness is also enhanced by larger wafer sizes required by modern technology thus leading to a very serious problem for the end use of said polished wafers for example the use of silicon polished wafers for large scale integrated (LSI) circuit manufacture and very large-scale integrated (VLSI) circuit applications. These applications require substantially flat polished wafer surfaces in order to achieve high resolution in the photolithographic steps of the integrated circuit manufacturing process.
Recent technological advances have enhanced methods of mounting the semiconductor slices to the carrier plate which allow the wafers to be subjected to operations including washing, lapping, polishing, and the like without mechanical distortion or unflatness of the polished wafers. For example, when utilizing the methodology for wax mounting of silicon wafers to carrier plates for further operations thereon, and particularly polishing to a high degree of surface perfection as appropriate for the manufacture of integrated circuits on such wafers, it has been observed that entrapped air bubbles in the wax layer under the slice create imperfections in the products which result from prior art methodology. Such imperfect methodology has been corrected by the invention disclosed and claimed in the recent Walsh U.S. application, Ser. No. 126,807, filed Mar. 3, 1980, entitled "Method and Apparatus for Wax Mounting of Thin Wafers for Polishing". The corrections afforded by Walsh's mounting methods are of little assistance in achieving uniform polished flatness of semiconductor wafers if the final polishing does not accommodate the continuation of uniform flatness. Modern requirements of the semiconductor industry regarding polished silicon wafers cannot tolerate surface flatness variations. In the manufacture of VLSI circuits, a high density of the circuit elements must be created on a silicon wafer requiring an extraordinarily high order of precision and resolution calling for wafer flatness heretofore not required. The necessary polished slice flatness for such applications, for example, less than about 2 micrometers peak to valley, cannot be achieved if the carrier mounted wafers are polished against a thermally-mechanically bowed polishing surface.